I am favourite to get American motivation (7)
I believe the answer is:
impetus
'motivation' is the definition.
(impetus can mean a motivation or stimulus)
'i am favourite to get american' is the wordplay.
'i am' becomes 'I'm' (contraction).
'favourite' becomes 'pet' (synonyms).
'to get' says to put letters next to each other.
'american' becomes 'us' (abbreviation for United States).
'im'+'pet'+'us'='IMPETUS'
(Other definitions for impetus that I've seen before include "Imputes to force with which a body moves" , "Incentive, stimulus" , "Thrust" , "Driving force, forward movement" , "Incentive - time's up (anag)" .)